The scheduler do that automatically for you. It runs once a day and compute what should be purchased, build (manufacturing orders or tasks) based on inventory levels and procurement rules.

If you do not want to wait 24 hours to see the result, you can launch the scheduler manually through the menu: Warehouse > Schedulers > Run Schedulers. (You may be in the "technical feature" mode to access this menu)

In order to compute what should be purchased and, in which quantity, OpenERP uses different mechanisms: